The present invention relates generally to the field of antenna systems. More particularly, the present invention relates to metrology antenna systems. The present invention is intended to serve as a reference radiator or receiver of electromagnetic radiation and provides a near perfect or canonical dipolar radiation pattern.
An antenna system consists of radiating/receiving elements as well as a feed network, which couples the radiating elements to an external device, such as an amplifier, or system, such as a receiver. A well-designed antenna system provides an input impedance that closely matches that of the external device or system to which the antenna system is connected at resonance. In this way reflections and standing waves are minimized. Thus, one task of the feed network can be to match the input impedance of the radiating element(s) to the impedance level of the system. Additionally, the feed network may convert a single-ended or unbalanced source into a balanced configuration. This is necessary if the antenna is a symmetric or balanced antenna such as a dipole and the source utilizes a coaxial port. Metrology antenna systems are one type of antenna system that, by design, should produce accurate and repeatable electromagnetic field measurements. Some electromagnetic field measurements include, but are not limited to site attenuation, anechoic chamber characterization, antenna characterization, in-situ telecommunication device characterization, and Specific Absorption Rate (SAR). In a metrology antenna system very little mismatch or imbalance can be tolerated. Therefore, the requirements for the feed network of a metrology antenna system are quite exacting. In order to provide the greatest possible confidence in measurements, it is desirable that a metrology antenna system be capable of being comprehensively modeled numerically or possibly analytically in a straightforward manner. In particular, it is desirable that the antenna system be designed such that well-established and extensively-verified numerical models such as the Numerical Electromagnetics Code (NEC-2, NEC-4) can be used to accurately model it. This limits the geometry of and the materials used in the antenna to those that can be accurately represented in the numerical model. In particular, the NEC code is extremely well adapted to representing linear antennas.
A linear antenna is essentially a one-dimensional antenna, that is, one that looks substantially like a linear wire. Linear antennas, include, but are not limited to, half-wave linear dipoles, quarter-wave linear monopoles, electrically-short linear dipoles, electrically-short linear monopoles, folded dipoles, folded monopoles, sleeve dipoles, and sleeve monopoles. FIG. 9 depicts an idealized center driven linear dipole antenna. As can be see in FIG. 9, the self-contained voltage source Vocos(xcfx89t) feeds the antenna system, which comprises two linear wire elements. Linear antennas are also referred to as wire antennas because sometimes they are fabricated from wire stock or other conducting materials. While it is possible to fabricate linear antennas from wire, such antennas are more often fabricated from rigid metal tubing or circular metal bar stock. One comprehensive reference on linear antennas is R. W. P. KING, THE THEORY OF LINEAR ANTENNAS WITH CHARTS AND TABLES FOR PRACTICAL APPLICATIONS, herein incorporated by reference in its entirety. One such linear antenna is shown in FIG. 9. FIG. 9 depicts a dipole antenna which utilizes a self-contained source. Most practical implementations of dipole antennas do not use a self-contained source. Instead, the antennas are driven via feed transmission lines. In most practical situations, this transmission line is a coaxial cable.
Even though a dipole is a symmetric antenna, it must be driven with a symmetric or balanced source, also known as a differential source in order to obtain a symmetric radiation pattern. As can be seen in FIGS. 10 and 11, the linear dipole, identified by two linear wire elements has a balanced source derived from two single-ended sources, namely the two xc2xd V0(cos(xcfx89t)) voltage sources. Schematic depictions of two equivalent, balanced sources are shown in FIG. 5. A balanced source produces two voltages equal in magnitude and opposite in phase with respect to a common reference. If the sources of a linear dipole are not symmetrically balanced, common mode current will flow on the feed transmission line. Common mode currents produce distorted radiation patterns and cross-polarized radiation thereby eliminating the principle benefit of the linear dipole, namely radiation patterns which are easily modeled. Some references describing these effects are in W. L. WEEKS, ANTENNA ENGINEERING, xc2xa74.5 (McGraw Hill 1968) and C. A. BALANIS, ANTENNA THEORY ANALYSIS AND DESIGN,xe2x80x9d xc2xa79.8.6 (John Wiley and Sons 1997) herein incorporated by reference in its entirety.
Accordingly, to prevent common mode currents which produce distorted radiation patterns, linear dipoles must be fed with balanced sources. Sources originally unbalanced may be converted to balanced sources using a BALanced-to-UNbalanced (BALUN) transformer or network. One simple example of a balun is a transformer with a center-tapped secondary, such as is shown in FIG. 6. With this configuration, as shown in FIG. 6, a single-ended source, V0(cos(xcfx89t)), is connected to the primary and the center tap of the secondary winding is connected to ground. It should be noted that V0 represents the magnitude of the AC voltage and xcfx89 represents its radian frequency. This produces two voltages equal in magnitude but opposite in phase, namely xc2xd V0(cos(xcfx89t)) and xe2x88x92xc2xd V0(cos(xcfx89t)). Linear dipole antennas are usually coupled to coaxial transmission lines through baluns. Some prior art baluns include, but are not limited to, the Marchand or Roberts balun, the choke balun, and the split sleeve balun. The Roberts dipole, a linear dipole driven by a Marchand balun, is a metrology standard and is specified in ANSI standard C63.5-1998, herein incorporated by reference in its entirety. These prior art baluns have a number of inadequacies. As for the Roberts balun these include: (1) calibration procedures using an automatic vector network analyzer are difficult to implement, (2) acceptable manufacturing tolerances for physically small devices such as are required for high frequency operations are difficult to achieve; and (3) spurious radiation from the balun which can significantly perturb the linear dipole""s radiation pattern. Choke baluns suffer from drawbacks similar to the Roberts balun. These shortcomings include: (1) the difficulty of implementing a calibration procedure using an automatic vector network analyzer; (2) physical limitations in magnetic materials, such as ferrite, which limit the operating frequency range of such devices to several GHz at the highest; and (3) spurious radiation from the balun.
As noted earlier, in order to increase confidence in electromagnetic field measurements, it is desirable to employ an antenna system that can be simply and accurately modeled analytically or numerically. This is particularly important for metrology or reference antenna systems. Because linear dipoles are among the simplest antenna structures and have been extensively analyzed, they are widely used in conjunction with metrology applications. Despite their simplicity, there are some difficulties encountered in the realization of practical dipole antennas. One difficulty involves the techniques used to feed the dipoles. Radiation originates from these feed mechanisms, and simple numerical and analytical models cannot accurately account for this radiation. Feed region radiation can cause the behavior of a practical dipole to depart markedly from that of an ideal or canonical dipole. Besides the distortion of the ideal canonical dipole radiation pattern caused by the feed source, radiation from the feed source also complicates the interaction of the antenna with its environment. That is, one of the desirable features of a linear dipole is its simple, low-order radiation pattern that allows straightforward prediction of its interaction with a scatterer such as a ground plane. However, spurious radiation from the feed region complicates the practical dipole""s radiation pattern and makes prediction of the interaction of the antenna with a scatterer such as a ground plane much more difficult. One prior art method used to prevent radiation caused by the feed regions involved reducing, the dimensions of the antenna feed regions. In order for the feed region not to affect radiation patterns, the feed regions needed to be approximately 0.01 of the size of the overall dipole length. This constraint is easier to satisfy for large dipole antennas such as at HF (3-30 MHz) and VHF (30-300 MHz) frequencies where wavelengths range from 10 meters down to 1 meter, however, for smaller antenna systems at, for example, 3000 MHz, a half-wave dipole is about 5 cm long. Creating a feed region, one hundredth the size of this dipole length is extremely difficult to manufacture and results in a large margin of error. This problem is exacerbated because the center feed region often provides the mechanical support for the dipole elements. To add further mechanical support often times a buttressing material such as shown in FIG. 13 is used to support and maintain the dimensions of the feed regions. However, this buttressing material, dielectric or otherwise, also alters the behavior of the antenna and therefore can cause a departure from canonical dipole behavior. Thus, it is desirable to minimize usage of such material.
A prior art technique for obtaining a shielded and hence a non-radiating balancing network involves the use of a shielded four-port 180-degree hybrid network. FIG. 8 depicts the four-port 180-degree hybrid network employed as a balun in the CALTS approach discussed below. FIG. 8 also demonstrates how the four-port 180-degree hybrid balun can be xe2x80x9cconnectorizedxe2x80x9d using standard coaxial connectors such as SMA or N connectors. With standard, 50xcexa9 coaxial connectors, the 180-degree, four port hybrid network can be easily characterized using automatic vector network analyzers. This is the procedure required in the CALibrated Test Site Amendment to the CISPR16-1 1993 (1999, International Electrotechnical Commission) (hereinafter xe2x80x9cthe CALTS approachxe2x80x9d) herein incorporated by reference in its entirety. The CALTS approach is depicted in FIGS. 8 and 14. While, the CALTS approach allows the antenna and balun to be xe2x80x9cconnectorizedxe2x80x9d and characterized using standard automatic vector network analyzers, the CALTS approach does not provide an effective way to prevent spurious radiation from the feed region. Hence, it is not easily adapted for use at higher frequencies. Moreover, the manner in which the 180-degree, four-port hybrid network is adapted as a balun places two of the ports (the 0xc2x0 and 180xc2x0 ports) in series thereby effectively doubling the port impedance. If the hybrid has 50xcexa9 coaxial ports, the effective 100xcexa9 impedance does not match the input impedance of the half-wave linear dipole which is 73-80xcexa9 depending on the diameter of the dipole. In the CALTS approach, this mismatch is remedied by the use of coaxial attenuators.
Prior art techniques, to match the effective source impedances with the dipoles impedance involved resistive matching pads placed between the 0 and 180-degree ports of the hybrid network and the dipole as shown in FIG. 8. The resistive matching pads effect an impedance match by dissipating power. While the resistive pads resulted in a matched network, the resistive pads also resulted in a reduced system gain. Specifically, the use of minimum loss pads to match a 100xcexa9 source such as a 50-xcexa9, 180-degree hybrid to the resistive 73-xcexa9 input impedance of a resonant, half-wave, linear dipole will result in a 5.00 dB loss in gain and therefore a xe2x88x923.14 dBi overall gain. Moreover, true minimum loss pads for this particular application are not commercially available. Instead, symmetric coaxial attenuators that are intended to work with equal source and load impedances (usually 50xcexa9) are commonly available. The CALTS specification calls for the insertion of such coaxial attenuators in between the 180-degree hybrid and the dipole antenna. However, the performance of such attenuators cannot be as good as that of a true minimum loss pad. For example, if 6 dB, 50xcexa9 attenuators were used in between a 180-degree hybrid and a resonant, linear half-wave dipole, the return loss would be xe2x88x9228 dB and the overall gain would be reduced by 6 dB to xe2x88x924.14 dBi. The signal-to-noise ratio of a measurement system incorporating two such dipole antenna systems (one for transmit and one for receive) would be decreased by 12 dB and, therefore, accurate electromagnetic field measurements would be difficult to achieve in some situations, especially those with high ambient noise levels. Furthermore, the use of resistive matching pads complicated high power antenna implementations such as needed for SAR measurements. In high power antenna implementations, power levels as high as 100 Watts are encountered. Matching pads or attenuators operating at this power level must be physically large in order to provide sufficient surface area for radiation of heat. Scattering from large matching pads further disturbs radiated field patterns. In principle, the impedance match can be implemented using reactive components, such as inductors and capacitors, and hence with minimal dissipative loss. However, practical inductors and capacitors exhibit significant tolerances and thus degrade the precision of the system.
One way to overcome the impedance matching problem between the 100xcexa9 effective output impedance of the 180 degree hybrid and the 73xe2x80x9480xcexa9 input impedance of the center-fed linear dipole is to essentially split the feed of the dipole and then symmetrically displace the two halves from the center as shown in FIG. 15. The effect of displacing the feeds from the center is an upward transformation or scaling of the impedance. This impedance transformation is given approximately by:             Z      in              Z      center        =            1      2        ⁢                  sec        2            ⁡              (                  kl          feed                )            
where Zcenter is the input impedance of the dipole when driven by a single source at the center, Zm is the driving point impedance seen at each of the two displaced sources, lfeed is the distance each source is displaced from the center, and k is the free space wavenumber associated with the electromagnetic field. By adjusting the distance of the feed regions from the center of the dipole, the impedance seen at each feed can be scaled upward from       Z    center    2
over a very wide range. Thus, in the case of a very thin linear, half-wave dipole the impedance may be easily transformed from             Z      center        2    =      36.5    ⁢          xe2x80x83        ⁢    Ω  
to 50xcexa9 in order to match the port impedances of a 180-degree 50-xcexa9 hybrid.
Accordingly, it would be advantageous to provide an improved linear dipole antenna system which can be accurately modeled numerically, utilizes a shielded balun which can be calibrated with an automatic network analyzer, the antenna having arbitrarily small feed regions and the antenna system being intrinsically matched to standard system impedance levels without using resistive matching pads or external matching networks.
The present invention eliminates one of the principal negative limitations associated with prior art dipole designs, which is, the need for resistive matching pads to match source impedances. Rather than using two resistive matching pads, the preferred embodiment of the present invention intrinsically matches the source impedance via the impedance transforming effects of a sleeve dipole antenna having two coaxial input ports and connected to a balancing network. The sleeve dipole antenna has two outer conductors and two inner conductors projecting from these two outer conductors which consequently create two arbitrarily small feed regions at the point where these inner conductors project. This two feed regions are then symmetrically displaced from a center point of the sleeve dipole antenna. By symmetrically displacing the two feed regions from the sleeve dipole antenna""s center point, the impedance of the sleeve dipole antenna at resonance increases. By shifting the feed regions, the sleeve dipole antenna""s impedance can be altered such that it matches the balancing network""s effective impedance at resonance.
In sum, the present invention provides an antenna system which essentially matches at resonance a particular source impedance without the use of an external matching network. In addition, the annular feed regions of the sleeve dipole antenna can be made arbitrarily small at little expense or with few manufacturing complications. Thus, the present invention creates an antenna with a highly predictable dipole radiation pattern without the use of matching pads. Moreover, because the annular feed regions can be made arbitrarily small, this antenna design is suited for high frequency implementations. High frequency implementations, as discussed above, require extraordinarily small feed regions to avoid distortion of the dipole""s radiation pattern.
One embodiment of the present invention includes an antenna system intrinsically matching a resistive impedance of a balancing network. The balancing network has a first output port and a second output port driven substantially one hundred and eighty degrees out of phase with respect to one another. The system includes a first transmission line connected to the first output port of the balancing network at a first end. This first transmission line has two ends, one connected to the balancing network as well as a free end. The first transmission line includes an inner conductor and a coaxially disposed outer conductor. In addition, the system includes a second transmission line. This second transmission line is also connected to the balancing network, however the second transmission line is connected to the second output port of the balancing network. Like the first transmission line, the second transmission line also has two ends. One end of the second transmission line is connected to the balancing network and a free end. The second transmission line extends from the balancing network co-linearly with respect to the first transmission line. The second transmission line also includes an inner conductor and a coaxially disposed outer conductor. The system further includes a sleeve dipole antenna having a resistive impedance at resonance. The sleeve dipole has a first input port and a second input port. The free end of the first transmission line is connected to the first input port of the sleeve dipole, while the free end of the second transmission line is connected to the second input port of the sleeve dipole. The sleeve dipole has two feed regions. The feed regions are displaced from the point of connection of the sleeve dipole to the first and second transmission lines. The feed regions are displaced so that the resistive impedance of the sleeve dipole at resonance matches the resistive impedance at the first and second output ports of the balancing network.
Another embodiment of the present invention involves an antenna system for connecting to a balancing network. This embodiment includes a first transmission line removably connected to a first output of a balancing network at a first end, this first transmission line having a free end. In addition, the embodiment includes a second transmission line removably connected to a second output of a balancing network at a first end, this second transmission line also having a free end. Finally, the embodiment includes a sleeve dipole antenna having a first coaxial input port and a second coaxial input port. The free end of the first transmission line is connected to the first coaxial input port of the sleeve dipole antenna while the free end of the second transmission line is connected to the second coaxial input port of the sleeve dipole antenna.
A further embodiment of the present invention includes an antenna system for connecting to a balancing network. This system includes a sleeve dipole antenna having a first inner conductor, a first outer conductor, a second inner conductor and a second outer conductor. The first inner conductor of the sleeve dipole antenna is coaxially disposed within the first outer conductor until a first feed region. This first feed region is created where the first inner conductor projects from the first outer conductor. The second inner conductor is also coaxially disposed within the second outer conductor until a second feed region. Like the first feed region, the second feed region is created where the second inner conductor projects from the second outer conductor. In addition, the system includes a first coaxial cable. The first coaxial cable connects and extends from a center point of the sleeve dipole antenna. In addition, the system further includes a second coaxial cable. This second coaxial cable connects and extends symmetrically with respect to the first coaxial cable from the center point of the sleeve dipole antenna.